Technical Field
This disclosure generally relates to active noise cancellation, masking, and suppression, for example, via at least one in-ear transducer in an ear canal of a user's ear.
Description of the Related Art
Active noise cancellation (ANC) is a noise control strategy using secondary, “anti-noise” sources to cancel out the primary, unwanted noise. It relies on the ability to generate acoustic waveforms having the same amplitude and opposite phase as compared to those of the primary noise, at every frequency of interest, in a “zone of quiet.” Within this zone, the primary and secondary sources interfere destructively, and the noise level is reduced.
Practical systems for ANC were first developed about 30 years ago and are now widely deployed, most commonly in the cabins of aircraft, automobiles, and heavy machinery, and as headsets for the mass consumer market. These systems typically provide 10-20 dB of active attenuation, concentrated at low frequencies (up to perhaps a few hundred Hz).
A typical ANC application is road- and engine-noise suppression in a luxury car. As passengers move, both their ear locations and the acoustic reverberation environment are constantly changing. The ANC system has direct knowledge of the sound waveforms only at strategically placed microphones in the headrests or walls. At low frequencies, the wavelength in air (340 meters/second divided by the frequency in Hz) is sufficiently long that these microphone signals are a good proxy for the actual heard waveforms, but at higher frequencies, a null at the microphone could easily be a peak in sound intensity in the user's ear. In such situations the anti-noise emitted by the system would make the original noise louder. Head-worn ANC systems have a feedback microphone in, e.g., the concha of the ear. Such is still around 3 cm from the wearer's eardrum, which is about one-tenth wavelength at 1000 Hz.